Interfacial Electron Transfer Followed by Photooxidation in N,N-Bis(p-anisole)aminopyridine-Aluminum(III) Porphyrin-Titanium(IV) Oxide Self-Assembled Photoanodes

Two self-assembled photoanodes have been constructed by exploiting the unique optical and structural properties of aluminum(III) porphyrin (AlPor) in conjunction with TiO₂ nanoparticles as an electron acceptor and bis(p-anisole)aminopyridine (BAA-Py) as an electron donor. AlPor is bound to the TiO₂ surface by either: (i) a benzohydroxamic acid bridge, in which the hydroxamic acid acts as an anchor or (ii) direct covalent binding of Al via an ether bond. The open sixth coordination site of the Al center is then used to coordinate BAA-Py through Lewis acid-base interactions, which results in donor-photosensitizer-semiconductor constructs that can be used as photoanodes. The two photoanodes were characterized by steady-state and transient spectroscopic techniques as well as computational methods. Transient-absorption studies show that in the absence of BAA-Py both the photoanodes exhibit electron injection from AlPor to the conduction band of TiO₂. However, the injection efficiencies and kinetics are strongly dependent on the linker with faster and more efficient injection occurring when the porphyrin is directly bound. Kinetic results also suggest that the recombination is faster in directly bound AlPor than benzohydroxamic acid bridged AlPor. When BAA-Py is coordinated to AlPor, electron injection from AlPor to TiO₂ is followed by electron transfer from BAA-Py to the oxidized AlPor. The injection efficiencies modeled using density functional theory and semiempirical tight-binding calculations are consistent with experimentally observed trends.